Avoid infinite loop when normalizer deletes characters

FIX: Fix an infinite loop in `SearchCursor` when the normalizer function
deletes characters.

See https://discuss.codemirror.net/t/modifying-tolowercase-function-to-support-diacritic-insensitive-search-causes-infinite-loop/8762

src/cursor.ts

@@ -74,7 +74,7 @@ export class SearchCursor implements Iterator<{from: number, to: number}>{
       let str = fromCodePoint(next), start = this.bufferStart + this.bufferPos
       this.bufferPos += codePointSize(next)
       let norm = this.normalize(str)
-      for (let i = 0, pos = start;; i++) {
+      if (norm.length) for (let i = 0, pos = start;; i++) {
         let code = norm.charCodeAt(i)
         let match = this.match(code, pos, this.bufferPos + this.bufferStart)
         if (i == norm.length - 1) {

test/test-cursor.ts

@@ -68,6 +68,10 @@ describe("SearchCursor", () => {
   it("produces the correct range for astral chars that get normalized to non-astral", () => {
     testMatches(new SearchCursor(Text.of(["𝜎"]), "𝜎"), [[0, 2]])
   })
+
+  it("can handle normalizers that remove text", () => {
+    testMatches(new SearchCursor(Text.of(["hello"]), "halal", 0, 5, s => s.replace(/[aeuoi]/g, "")), [[0, 4]])
+  })
 })
 
 describe("RegExpCursor", () => {